This invention relates to switching voltage convertors, and, more particularly, to switching voltage convertors for providing a constant dc voltage from the voltage available on the subscriber loop of a telephone network.
In recent years there have been significant advances in the development of equipment to be used in conjunction with residential and business telephone services. Such advances include add-on devices such as low-cost speaker phones, automatic dialers, automatic answer machines, and the like. In addition, the International Telegraph and Telephone Consultative Committee (CCITT) has recently specified a four-wire "S" interface standard for digital customer access (DCA) in an Integrated Services Digital Network (ISDN) which allows direct digital communications as well as improved digitized voice communications within a customer's residential or business facility. Devices which have been designed to work with this improved interface have sophisticated analog and digital circuits which require well-regulated voltages.
Typically, the add-on devices are electrically connected to the subscribers' telephone lines to receive and transmit signals thereon. A number of voltage convertor devices have been developed for providing the power to the add-on devices from the telephone lines. These voltage convertor devices take advantage of the fact that a typical subscriber telephone line provides a voltage differential of approximately 40 volts across the telephone lines when the receiver is on hook. Therefore, this voltage can be used to provide a source of dc power during the on hook condition. Typically, the add-on devices have internal batteries which are charged during the on hook condition to provide a continuing source of dc power when the receiver is off hook.
A typical voltage requirement for an exemplary add-on telephone circuit is 5 volts dc. This 5 volt source of power must be derived from the nominal 40 volts between the subscriber lines. Generally, a further requirement is imposed that the power tapped from the subscriber telephone loop be maintained at or below 400 mW. In order to provide a maximum amount of power at the 5 volt level for the add-on circuits, it has been found to be particularly advantageous to use a switching voltage convertor to provide the voltage for the add-on circuits. Although switching voltage convertors are well known to the art, it has been found that the convertors generally available suffer from problems with regulation. That is, although the differential voltage on the telephone subscriber loop is nominally 40 volts, fluctuations in the loading of the subscriber networks can cause the voltage to vary such that it is as low as 20 volts and as high as 48 volts. Thus, the switching voltage convertors must be designed to provide a nominal output of 5 volts while the input voltage to the convertor varies by more than 20 volts. Thus, a switching voltage convertor for use with a telephone subscriber network must be capable of suppressing variations in the output voltage over a wide range of variations in the input voltage.
Various apparatus and methods have been developed in the art to suppress variations in the output voltage of switching voltage convertors. However, the apparatus are typically complex and require a large number of control components, many of which cannot be incorporated into a single integrated circuit. Thus, a need exists to provide a control circuit for a switching voltage convertor for providing a constant output voltage over a wide range of input voltages.